1. SRM INSTITUTE OF SCIENCE AND TECHNOLOGY
Delhi-NCR Campus, Ghaziabad
Subject Name-Electromagnetic Theory, Quantum Mechanics, Waves
and Optics
Subject Code: 21PYB101J
Module I- DIELECTRICS
Video No-03
By
Dr. Megha Gupta Chaudhary
Assistant Professor, Physics
2. Dielectric Materials
Dielectric materials are also called as insulators.
In dielectric materials, all the electrons are tightly bound to their parent
molecules and there are no free charges. In addition, the forbidden energy
band gap (e.g.) for dielectric materials is more than 3eV.
Not possible for the electrons in the valence band to excite to the conduction
band, by crossing the energy gap, even with normal voltage or thermal energy.
3. The dielectric materials can be classified into active and passive dielectric
materials.
i. Active dielectrics
When a dielectric material is kept in an external electric field, if it
actively accepts the electricity, then it is known as active dielectric
material. Thus, active dielectrics are the dielectrics, which can easily
adapt themselves to store the electrical energy in it.
Eg: Piezoelectric and ferroelectric materials.
ii. Passive dielectrics
Passive dielectrics are the dielectrics, which restrict the flow of
electrical energy in them. So, these dielectrics act as insulators.
Examples: All insulating materials such as glass, mica, rubber etc.,
4. i. Non-Polar Dielectrics
There is no permanent dipole existence in the absence of electric
field
Eg: H2, N2, O2, CO2
ii. Polar dielectrics
There is existence of permanent dipole in the absence of electric filed
Examples:H2O, HCI, CO
5. Basic Definitions in Dielectrics
Electric Field
The region around the charge within which its effect is felt or experienced is known
as electric field.
The electric field is assumed to consist of imaginary electric lines of force. These
lines of force originate from the positive charges and terminate to the negative
charges .
Electric field strength or electric field intensity (E)
Electric field strength at any point is defined as the force experienced by an unit
positive charge placed at the point. It is denoted by ‘E’.
‘q’ - magnitude of the charge in coulombs
‘F’ - force experienced by that charge in Newton,
electric field strength (E) = F/q
⮚Its unit is Newton / Coulomb (or) volt / meter.
6. Basic Definitions in Dielectrics
Electric flux (Φ)
It is defined as the total number of electric lines of force passing through a given
area in the electric field.
Unit: newton meters squared per coulomb (Nm2C-1)
Φ=E.A=EACosθ
Electric flux density or electric displacement vector (D = Φ /A)
It is defined as the number of electric lines of force passing normally through an unit
area of cross section in the field.
Its unit is Coulomb / m2
7. Dielectric Constant
The dielectric constant or relative permittivity of a material determines its dielectric
characteristics. It is the ratio of the permittivity of the medium and the permittivity of
free space
εr = ε/ε0
It is also defined as the capacitance of the capacitor with dielectric to capacitance of
the same capacitor without dielectric.
•It is a measure of the electric polarization in the dielectric material and it has no units.
•Materials with higher dielectric constant are easily polarized and behaves as good
electrical insulators.
0
C
C
K
8. Electric Polarization (The process of creating or inducing diploes in DEM)
⮚ Consider an atom. We know that it is electrically neutral. Furthermore, the centre of
the negative charge of the electrons coincides with the positive nuclear charge, which
means that the atom has no net dipole moment.
⮚ However, when this atom is placed in an external electric field, the centre of the
positive charge is displaced along the field direction while the centre of the negative
charge is displaced in the opposite direction.
⮚ When a dielectric material is placed inside an electric field, such dipoles are created in
all the atoms inside.
Polarizability (α)
When the electric field strength ‘E’ is increased, the strength of the induced dipole is
also increased. Thus, the induced dipole moment is proportional to the intensity of the
electric field.
p ∝ E
p = αE
9. Polarization vector
The dipole moment per unit volume of the dielectric material is called polarization
vector.
P = Np
Unit: Coulomb / m2
‘p ’ - average dipole moment per molecule and
‘N’ - number of molecules per unit volume
Relation between P, ε0 , ε r and E
The polarization ‘P’ is related to the electric flux density D as,
D = ε0E + P
Since D = ε0εr E, the above relation becomes,
ε0 εr E = ε0E + P
(or) P = ε0εr E− ε0E
i.e. P = ε0 (ε r − 1 )E
10. Electric susceptibility or Dielectric Susceptibility ( ):
The dielectric susceptibility is defined as the ratio of polarization to the net field εoE
modified by the induced charges at the surface.
When a dielectric is placed in an external electric field E, the polarization P is always
proportional to the electric field. That is
1
)
1
(
0
0
0
0
r
e
r
e
e
E
E
E
P
E
P
11. Various Polarization mechanisms in Dielectrics
Dielectric polarization is the displacement of charged particles under the action of the
external electric field. Several microscopic mechanisms are responsible for electric
polarization.
Four types of microscopic polarization mechanisms.
Electronic polarization
Ionic polarization
Orientation polarization and
Space-charge polarization.
12. Electronic Polarization: This also called atom or atomic polarization.
It is defined as an electric strain produced in an atom by the application of electric field.
It results from the displacement of nucleus (+Ze) and electrons (-Ze) in opposite direction in the
presence of the applied field with the creation of dipole moment.
The dipole moment induced is proportional to the strength of field applied
p ∝ E
p = αE
where αe is electronic polarizability of the material which is given by
N is number of atoms per cm3
Polarization is
It is independent of temperature.
Monoatomic molecules exhibits this
type of polarization.
N
r
o
)
1
(
E
N
P e
e
13. Ionic polarization
In this case a (solid) material must have some ionic character.
It then automatically has internal dipoles, but these built-in dipoles exactly cancel
each other and are unable to rotate. The external field then induces net dipoles
by slightly displacing the ions from their rest position.
This is observed in the materials that posses symmetric molecules like NaCl.
It does not occur in typical covalent crystals such as diamond.
Ionic polarization is independent of temperature.
The ionic polarization is
m
M
E
e
Pi
1
1
2
0
2
14. Orientation polarization
It is due to the presence of polar molecules in the dielectric material which have permanent dipole
moment.
In thermal equilibrium, the dipoles will be randomly oriented and thus carry no net polarization. The
external field aligns these dipoles to some extent and thus induces a polarization of the material.
Example:- H2O in its liquid form.
It occurs in asymmetric molecules.
Its depends on the temperature.
, μ is permanent dipole moment
KT
Po
3
2
15. Space-charge polarization
It occurs due to the accumulation of electric charges at the interface of a multi
phased material.
This is possible when one of the phases present posses much higher resistivity
than the other.
It is found to occur in ferrites and semiconductors.
The total polarization is P=Pe+Pi+Po+Ps
16. Since the space-charge polarizability is very small when compared
to the other types of polarizabilities, the total polarizability of a gas
can be written as
17. FREQUENCY DEPENDENCE OF POLARIZABILITY
On the application of alternating electric field the polarization process occur as a
function of time.
Electronic polarizability is extremely rapid and is complete at any instant of time even
when the frequency of the voltage is very high in the optical ranges. Thus it occurs at
all frequencies.
Ionic polarizability is slower and the ions do not respond at all when the voltage
correspond to visible frequencies. So it does not occur at visible frequencies.
Orientational polarization is slower than the ionic polarizability and occurs only at
frequencies which are smaller than the infrared frequencies.
Space charge polarization is slower process and occurs only at lower frequencies (50-
60 Hz).
The total polarizability is very high at low frequencies and very low at higher (optical)
frequencies.
